In continuous ink jet printing, graphic reproduction is accomplished by selectively charging and deflecting drops from the drop streams and depositing at least some of the drops on a print receiving medium while other of the drops strike a drop catcher device. In order to provide precise charging and deflecting of the drops, it is important that the drop break-up process produce uniformly sized and timed drops. Drop generators for such printers produce the required drop formation by vibrating the orifices from which the ink emerges.
In U.S. Pat. No. 4,473,830, a feedback transducer was utilized for monitoring the vibration amplitude. The drive amplitude for the drop generator was servo controlled to maintain the proper feedback amplitude. Co-pending, commonly assigned U.S. patent application Ser. Nos. 796,490 and 858,796, filed Nov. 22, 1991, and Mar. 27, 1992, respectively, incorporated herein by reference, further employ this feedback signal to insure that the drive signal tracks the resonance of the drop generator and to provide the proper phasing of the drop charging electronics with respect to the stimulation.
These stimulation control means function appropriately when the feedback signal has a sufficient signal-to-noise ratio. In the field of ink jet printers, however, it is desirable from the standpoint of throughput to utilize long arrays of ink jets. Unfortunately, the signal-to-noise ratio tends to be degraded in ink jet printers having long arrays and high resonant frequency. Long arrays of jets have more charging electrodes which must be rapidly switched between ground potential and high voltage. These switching transients produce electrical noise which can be picked up at the input to the stimulation control electronics. Even with careful shielding and filtering, the electrical noise picked up by the stimulation control electronics can affect the stimulation drive amplitude and frequency. Higher frequency drop generators tend to have lower feedback signal amplitudes, further reducing the signal-to-noise ratio.
It is seen then that there is a need for a means of improving the signal-to-noise ratio of the feedback signal used by the stimulation control electronics.